1. Field of the Invention
The present invention relates to a process for recovering and treating fluids such as hydrocarbon gas and/or liquids, and produced water from a hydraulic fracturing process.
2. Description of the Related Art
The technique of hydraulic fracturing is used to increase or restore the rate at which fluids, such as oil, water, or natural gas can be produced from subterranean natural reservoirs. Reservoirs are typically porous sandstones, limestones or dolomite rocks, but also include reservoirs such as shale rock or coal beds.
A hydraulic fracture is formed by pumping fracturing fluid into the wellbore at a rate sufficient to increase pressure downhole to exceed that of the fracture gradient of the rock. The rock cracks and the fracture fluid continues farther into the rock, thereby extending the crack.
The fluid injected into the rock is typically a slurry of water, proppants, and chemical additives. Additionally, gels, foams, and compressed gases, including nitrogen, carbon dioxide and air can be injected. Various types of proppant include silica sand, resin-coated sand, and man-made ceramics. These vary depending on the type of permeability or grain strength needed. Chemical additives are applied to tailor the injected material to the specific geological situation, protect the well, and improve its operation, though the injected fluid is approximately 98-99.5% percent water, varying slightly based on the type of well. The composition of injected fluid is sometimes changed as the fracturing job proceeds. Often, acid is initially used to scour the perforations and clean up the near-wellbore area. Afterward, high pressure fracture fluid is injected into the wellbore, with the pressure above the fracture gradient of the rock. This fracture fluid contains water-soluble gelling agents (such as guar gum) which increase viscosity and efficiently deliver the proppant into the formation. As the fracturing process proceeds, viscosity-reducing agents such as oxidizers and enzyme breakers are sometimes then added to the fracturing fluid to deactivate the gelling agents and encourage flowback. The purpose of the proppant is primarily to provide a permeable and permanent filler to fill the void created during the fracturing process. At the end of the job the well is commonly flushed with water (sometimes blended with a friction reducing chemical) under pressure.
The injected fluid is to some degree recovered and is managed by several methods, such as underground injection control, treatment and discharge, recycling, or temporary storage in pits or containers. Although the concentrations of the chemical additives are very low, the recovered fluid may be harmful due in part to hydrocarbons and other species (for example, hydrogen sulfide) picked up from the formation.
The technology in use today for treating recovered hydraulic fracturing fluids has been adopted from oilfield and conventional wastewater treatment technologies. The design of the system and the unit operations deployed depend upon the requirements of the water to be treated and the regulations in force in the locale where fracturing is taking place. Commercially available technologies that have been deployed may include:
De-gassing unit operations including flash pressure separations;
De-oiling technologies that use gravity separation such as skim tanks, flotation separation processes, and oil adsorption filters sometimes used in conjunction with water-in-oil emulsion breakers and oil-in-water emulsion breakers;
Removal of suspended solids through technologies such as agglomeration, settling, filtration, hydrocyclones, or centrifugation;
Removal of dissolved chemical species through processes such as coagulation & flocculation followed by physical separation such as settling, filtration, hydrocyclones, or centrifugation; oxidation of dissolved species with chemicals or light; absorption or adsoprtion, ultra- or nano-filtration; and evaporation.
Removal of bacterial activity through disinfection chemistries, light, and/or filtration.